Abstract
Recently, the hot stamping process using local blank heating has been widely used to manufacture lightweight and crashworthy automotive parts. However, the hardness prediction of hot stamped parts produced using local blank heating is difficult because it involves many process variables, such as the heating temperature, heating time, and cooling rate. The purpose of this study was to predict the hardness of hot stamped parts fabricated using local blank heating based on quench factor analysis (QFA). The volume fraction of austenite was measured to consider the phase transformation in the heating stage, and it was expressed by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Additionally, a dilatometry test was performed to measure the hardness according to the cooling rates, which was used to determine the material constants for QFA. Finite-element simulation was performed to predict the temperature histories during the hot stamping process and the results were used to predict the hardness according to QFA with the JMAK equation. A hot stamping experiment with local blank heating equipment was performed, and the predicted and experimental results were compared for verification of the proposed hardness prediction method.
Highlights
Reducing the car body weight has emerged as an important issue in the automotive industry because of the strengthened fuel efficiency and emission regulations [1,2]
FE simulation was performed to predict the temperature histories during the hot stamping process and the results were used to predict the hardness according to quench factor analysis (QFA) with the JMAK equation
In the hot stamping process with local blank heating, the volume fraction of austenite according to the heating condition significantly affects the mechanical properties of the final products; it should be accurately predicted
Summary
Reducing the car body weight has emerged as an important issue in the automotive industry because of the strengthened fuel efficiency and emission regulations [1,2]. The hot stamping process is widely used to manufacture automotive parts for reducing the car body weight and achieving high formability and excellent dimensional accuracy [3,4]. Hot stamped parts are unsuitable as crash absorbing parts because of their high strength and low ductility. To solve these problems, the hot stamping process with a tailored blank is used in the manufacturing of car body parts [5]. Local blank heating means that the complete thermal cycle above austenitization would be applied only in high-strength region, whereas the other zone would be heated below the Ac3 temperature to control the phase transformation of austenite [6]. The microstructure of partial austenite is transformed into martensite, and excellent ductility is achieved during the hot stamping process
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